1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as a resonator or a band filter in communication devices or other apparatuses, and more particularly, the present invention relates to an edge reflection type surface acoustic wave device which utilizes an SH type surface acoustic wave.
2. Description of the Related Art
Edge reflection type surface acoustic wave devices which utilize an SH type surface acoustic wave such as a BGS (Bleustein-Gulyaev-Shimizu) wave, a Love wave, or other such waves are known. In an edge reflection type surface acoustic wave device, edges are formed which are on the opposite ends in the surface acoustic wave propagation direction of the area where an interdigital transducer (IDT) is provided, and the edges are perpendicular to the surface acoustic wave propagation direction. A surface acoustic wave is reflected by the edges. Accordingly, reflectors are not necessary and as a result, the surface acoustic wave device can be miniaturized.
Japanese Unexamined Patent Publication No. 7-263998 discloses an edge reflection type surface acoustic wave resonator in which grooves are formed on a surface acoustic wave substrate, and the inner side walls of the grooves constitute the reflection edges. As shown in FIG. 7, in an edge reflection type surface acoustic wave resonator 51, an IDT 53 is formed on a surface acoustic wave substrate 52. On the opposite ends in the surface acoustic wave propagation direction of the IDT 53, grooves 52a and 52b are formed. The inner side walls 52a1 and 52b1 of the grooves 52a and 52b constitute the reflection edges.
The inner side walls 52a1 and 52b1 of the grooves 52a and 52b are used as the reflection edges because the stability of mounting is improved. In the edge reflection type surface acoustic wave resonator 51, only one IDT 53 is provided. When the number of pairs of the electrode fingers of the IDT 53 is small, the chip size in the surface acoustic wave propagation direction is very small, so that the mounting stability is deteriorated. Accordingly, substrate portions 52c and 52d are formed on the outer sides of the inner side walls 52a1 and 52b1 constituting the reflection edges, and thereby, the chip size is increased in the surface acoustic wave propagation direction, so that the mounting stability is improved.
On the other hand, there are some edge reflection type surface acoustic wave devices which have a relatively large number of electrode finger pairs such as an edge reflection type longitudinally coupled SAW resonator filter having a plurality of IDTs. For example, as shown in FIG.8, in an edge reflection type longitudinally coupled SAW resonator filter 61, two IDTs 63 and 64 are formed on a surface acoustic wave substrate 62. In this case, since the IDTs 63 and 64 are provided, the number of electrode fingers is relatively large, and thereby, the distance between the side surfaces 62a and 62b of the surface acoustic wave substrate 62 is relatively large. Thus, the chip size measured in the surface acoustic wave propagation direction is sufficiently large, and thereby, the mounting stability is high.
That is, for an edge reflection type longitudinally coupled SAW resonator filter, it is not necessary to form the substrate portions 52c and 52d on the outer sides of the grooves 52a and 52b, respectively, in contrast to the surface acoustic wave resonator 51 shown in FIG. 7. If the substrate portions 52c and 52d are provided, they will prevent miniaturization without serving any useful purpose.
In the edge reflection type surface acoustic wave device, the electrode fingers of the IDTs 63 and 64 are located near the side surfaces 62a and 62b, similarly to the edge reflection type longitudinally coupled SAW resonator filter 61. That is, the electrode fingers 65a and 65b located at the outermost ends in the surface acoustic wave propagation direction are formed along the edges defined by the side surfaces 62a, 62b and the upper surface 62c, respectively.
Referring to the production of the SAW resonator filter 61, the IDTs 63 and 64 are formed on a mother surface acoustic wave substrate, and thereafter, the surface acoustic wave substrate is cut in the thickness direction to form the side surfaces 62a and 62b. The side surfaces 62a and 62b should be formed with high precision, since the edges defined by the side surfaces 62a, 62b reflect a surface acoustic wave. In addition, since the electrode fingers 65a and 65b are located near the edges 62a and 62b, respectively, chipping at the surface of the surface acoustic wave substrate 62 which is caused during cutting of the substrate 62 should be prevented as much as possible. If the chipping and the breaking of a substrate material is increased, the electrode fingers 65a and 65b will be disconnected, which will considerably affect the filter characteristics.
Such problems as described above arise not only in the edge reflection type longitudinally coupled type SAW resonator filter 61 but also in other edge reflection type surface acoustic wave devices such as an edge reflection type surface acoustic wave resonator.
On the other hand, Japanese Unexamined Patent Publication No. 9-326373 discloses a method of preventing the chipping of a substrate, which occurs during cutting of a semiconductor wafer or other such component. In particular, as the method of cutting a substrate, a bevel cutting method illustrated in FIG. 9A, and a step cutting method shown in FIG. 9B are described.
As shown in FIG. 9A, according to the bevel cutting method, first, a V-shaped groove 71a is formed on a substrate 71 via a blade 72 having a V-shaped cross-section. Next, the portion of the substrate 71 where the V-shaped groove 71a is formed is cut via another cutting blade 73 so that the groove extends to and reaches the lower major surface 71b of the substrate 71. According to this method, when the cutting is performed via the blade 73, chipping at the edge defined by the upper surface 71d and the side 71c of the substrate 71 is prevented, since the V-shaped groove 71a is previously formed.
Further, according to the step cutting method shown in FIG. 9B, first, the substrate 71 is imperfectly cut via a cutting blade 74 having a relatively wide cutting width to form a groove 71e such that a small cutting margin is left. Next, cutting is performed in the groove 71e with a blade 75 having a relative narrow cutting width, so that the groove extends to and reaches the lower surface of the substrate 71. In this case, the second cutting step can be easily performed, since the cutting is carried out from the bottom of the groove 71e to the lower surface 71b of the substrate 71 via the blade 75. Thus, chipping at the lower surface 71b of the substrate 71 can be prevented.
However, in an edge reflection type surface acoustic wave device, when reflection edges are formed by cutting, it is necessary not only to prevent the above-described chipping but also to form the reflection edges which are as vertical relative to the substrate as possible. Thus, according to the bevel cutting method, an inclined-surface portion is formed between the upper surface 71d of the substrate 71 where the IDT is formed and the side surface 71c thereof, which is caused by the V-shaped groove 71a. Therefore, the verticality of the upper portion of the side is deteriorated.
On the other hand, the step cutting method is effective in preventing chipping at the lower surface of a substrate, but is not effective in preventing chipping at the upper surface of a substrate.
To overcome the problems described above, preferred embodiments of the present invention provide an edge reflection type surface acoustic wave device and a method of manufacturing thereof, in which reflection edges are formed with high precision so as to be perpendicular to a major surface of a surface acoustic wave substrate on which an IDT is provided, so as to prevent disconnection of an electrode finger caused by chipping and so that the surface acoustic wave device has excellent resonance characteristics and filter characteristics.
An edge reflection type surface acoustic wave device which utilizes an SH type surface acoustic wave according to a first preferred embodiment of the present application preferably includes a surface acoustic wave substrate, and at least two IDTs provided on one main surface of the surface acoustic wave substrate, wherein first and second grooves are formed on the opposite ends in the surface acoustic wave propagation direction of the area where the IDT is provided, so as to extend from the one main surface of the surface acoustic wave substrate toward the other main surface thereof while not reaching the other main surface and so as to extend substantially perpendicularly to the surface acoustic wave propagation direction, and first and second edges for reflecting the surface acoustic wave, the first and second edges being defined by the inner side walls of the first and second grooves, respectively.
The first and second grooves can be formed by cutting the surface acoustic wave substrate beginning from one main surface of the surface acoustic wave substrate and extending toward the other main surface of the surface acoustic wave substrate while not reaching the other main surface. Therefore, the grooves are formed with high precision substantially without chipping of the surface acoustic wave substrate and disconnection of the electrode fingers of the IDTs. Accordingly, an edge reflection type surface acoustic wave device having excellent resonance characteristics and filter characteristics is achieved.
When the electromechanical coupling coefficient of thickness shear vibration is represented by k15, and the wavelength of a surface acoustic wave by xcex, the first and second grooves are preferably formed so as to have a depth H satisfying the following formula (1):
0.2xe2x89xa7exp (xe2x88x922xcfx80k152H/xcex)xe2x80x83xe2x80x83(1)
It is further preferable that the depth H of each of the first and second grooves is not less than the wavelength 80, of a surface acoustic wave to be generated in the surface acoustic wave device.
In the case of the depth H of the first and second grooves formed so as to satisfy the formula (1), and in the case of the depth H of the grooves not less than the wavelength xcex of a surface acoustic wave, according to preferred embodiments of the present invention, most of the energy of the surface acoustic wave is reliably and securely reflected from the inner side walls of the grooves. Accordingly, an edge reflection type surface acoustic wave device having greatly improved resonance characteristics and filter characteristics is achieved.
According to the another preferred embodiment of the present invention, an edge reflection type surface acoustic wave device which utilizes an SH type surface acoustic wave, includes a surface acoustic wave substrate, and at least one IDT provided on the surface acoustic wave substrate, wherein steps are provided at the positions having an intermediate height of the first and second side surfaces positioned at the outer edges in the surface acoustic wave propagation direction of the surface acoustic wave substrate, respectively, and first and second edges for reflecting the surface acoustic wave are defined by the portions of the side surfaces located above the steps in the first and second side surfaces, respectively.
The above-described steps can be formed by cutting or splitting the portions below the bottoms of the grooves after the grooves are formed to extend from one main surface of the surface acoustic wave substrate without reaching the other main surface, respectively. Accordingly, during the formation of the grooves, chipping and the disconnection of the electrode fingers of the IDTs are reliably prevented. Thus, according to this preferred embodiment, an edge reflection type surface acoustic wave device having excellent resonance characteristics and filter characteristics is achieved.
When the electromechanical coupling coefficient of thickness shear vibration is represented by k15, and the wavelength of a surface acoustic wave by xcex, the first and second edges are preferably formed so as to have the distance D between the one main surface of the surface acoustic wave substrate and each of the steps, which satisfies the following formula (2).
0.2xe2x89xa7exp (xe2x88x922xcfx80k152D/xcex)xe2x80x83xe2x80x83(2)
In addition, it is preferable that the distance D between the one main surface of the surface acoustic wave substrate and each step is not less than the wavelength xcex of a surface acoustic wave.
When the distance D between the one main surface of the surface acoustic wave substrate and each step is formed so as to satisfy the formula (2), or when the distance D is not less than the wavelength xcex of a surface acoustic wave, most of the energy of the surface acoustic wave is reliably and securely reflected from the side surface portions located above the steps, respectively. Thus, an edge reflection type surface acoustic wave device having even more improved resonance characteristics and filter characteristics is achieved.
The unique features and structure of the edge reflection type surface acoustic wave devices described above can be applied to various edge reflection type surface acoustic wave devices such as an edge reflection type surface acoustic wave resonator, an edge reflection type surface acoustic wave filter, and other such devices.
For example, in the case of the longitudinally coupled type SAW resonator filter including the plural IDTs arranged in the surface acoustic wave propagation direction, a longitudinally coupled type SAW resonator filter having excellent filter characteristics is achieved. A transversely coupled resonator filter having excellent filter characteristics is achieved by arranging plural IDTs perpendicularly relative to the surface acoustic wave propagation direction.
Further, a ladder type filter having excellent filter characteristics is provided by forming plural IDTs on a single surface acoustic substrate to produce plural edge reflection type surface resonators, and electrically connecting the resonators via a connecting conducting part so as to form a ladder type filter.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.